Heavy polynuclear aromatic (HPNA) compounds may be a secondary byproduct from a hydrocracking process. The HPNA compounds can be a problem particularly for high conversion hydrocracking units, and be present in the reactor product. Recycling unconverted oil to increase yields of distillate product can result in an accumulation of HPNA compounds in the recycled oil. Accumulated HPNA compounds in the recycle oil may deposit on the catalyst as coke, which may degrade catalyst performance and result in shorter catalyst cycle length. Production of undesired HPNA compounds can be more pronounced for hydrocracking units processing heavier feeds. Thus, it would be desirable to remove the HPNA compounds from the unconverted oil so as to minimize the catalyst deactivation.
One way to remove HPNA's is to lower conversion by bleeding a portion of the unconverted oil to limit the accumulation of HPNA compounds. Unfortunately, this is often undesirable due to economical and logistical considerations because of yield loss and lack of demand for the unconverted oil. In order to minimize the bleed rate of unconverted oil, schemes such as carbon bed absorption of the recycle oil stream to remove HPNA's and stripping columns to concentrate the HPNA's in an unconverted oil waste stream have been commercially implemented. Stripping sections for removing HPNA's have been located in the product fractionation column or in a separate stripping column downstream of the product fractionation column. Used stripping steam carrying the HPNA lean stream from the stripper have been recycled back to the product fractionation column to provide stripping steam requirements. However, no solution has been offered for removing HPNA's from a bottoms stream from a product fractionation column that utilizes a reboiler.
It would be desirable to remove HPNA's from a fractionated stream exiting a product fractionation column that utilizes a reboiler for column heat requirements.